arm64: dts: freescale: imx{91,93}-phycore-som: Set BUCK5 in FPWM mode

arm64: dts: freescale: imx{91,93}-phycore-som: Set BUCK5 in FPWM mode

Set PMIC BUCK5 mode to forced PWM (Pulse Width Modulation) mode instead
of the default automatic PFM and PWM transition mode. FPWM mode produces
less ripple on the output voltage rail under light load conditions. And
since BUCK5 supplies SoC internal ADC reference voltage we need to keep
voltage ripple to a minimum. This solves issues with the occasional ADC
calibration procedure failures on phyCORE-i.MX91/93 SoM based boards.

Signed-off-by: Primoz Fiser <primoz.fiser@norik.com>
Signed-off-by: Frank Li <Frank.Li@nxp.com>